Multi-layer coating systems have been used to coat automotive vehicles for a number of years, but the early development of these systems necessarily employed systems based upon organic solvents ("solvent-borne" systems). As environmental concern over the use of volatile organic solvents has grown and the cost of such solvents has increased, solvent-borne coating systems have become less desirable. Recent research efforts in the coatings art have therefore focused on the development of water-borne coating systems.
The shift from organic solvents to water for dispersing and applying the resins, pigments and other components of a coating system has addressed many of the environmental and cost concerns of solvent-borne systems, but has at the same time introduced problems peculiar to water-borne coatings. One such problem relates to the use of micaceous particulates in water-borne coating systems to achieve so-called "metallic" effects in automotive coatings.
Metallic effects are generally achieved in automotive coatings by incorporating into the pigmented base coat composition of a multi-layer coating system of highly reflective, finely divided particulates. The particulates are generally aluminum flake, mica particles, or mica particles which have been encapsulated or coated with a metal oxide, typically iron oxide or titanium dioxide. Distribution of the finely divided reflective particulates throughout the cured base coat layer produces a metallic sparkle effect which is popular with the automotive consuming public.
However, mica particulates and metal oxide encapsulated or coated mica particulates do not disperse well in water-borne coating systems. This problem affects both the formulation and storage of the wet coating compositions and the appearance of the finished coating. In formulating wet coating compositions, often special processing considerations must be given to insuring the uniform incorporation of micaceous particulates to avoid aggregation of the particles. The resulting compositions are also frequently unstable, having short shelf lives. Precipitation of the micaceous particulates from the wet coating compositions results in hard, dry deposits of micaceous particulates in the bottom of containers. These deposits cannot usually be stirred back into the coating formulation, and the batch must be discarded. To counter this latter problem, it is often necessary to make up water-borne coating compositions just prior to use.
In base coat layers deposited from water-borne coating compositions containing micaceous particulates, the desired orientation of the platelet faces generally parallel to the base coat surface is frequently not optimized. The particles often orient at random angles which deviate in varying amounts from parallel to the base coat surface. When the particles have longitudinal dimensions on the order of 50 .mu.m, the particles are longer than typical base coat layer thicknesses. Particles which are oriented at rakish angles will thus protrude through the upper surface of the base coat, contributing to an undesirable generally "fuzzy" appearance to the finished coating. Moreover, when the particles do orient generally parallel to the base coat surface, there is often a tendency of the particles to congregate near the bottom surface of the base coat layer, i.e. the surface nearest the substrate. In these cases, colored pigments in the base coat layers can hide or mask a fraction of the mica particles, diminishing to some extent the desired metallic effect of the coating.
There is thus a need in the coatings art of a means of overcoming these problems when micaceous particulates are employed in water-borne coatings systems.